The majorities of human cancers show no obvious inheritance pattern, but appear to develop through a multistep process in which genetic changes alter the normal control over cell division. A multitude of factors, including viruses, exposure to ultraviolet radiation and carcinogens, may contribute to a weakening of checkpoint controls in cell division. Also the aging process involves cumulative oxidative damage to gene promoters and a selective reduction in expression of mitotic regulators and centrosome proteins including cyclins A, B, F and PLKs. In combination, an incremental loss of checkpoint controls increases the probability of mitotic errors over multiple cell divisions that in combination with activating mutations in proto-oncogenes and loss of tumour suppressors lead to the progression of benign lesions into malignant tumours.
Polo-like kinases (PLKs) are key enzymes that control mitotic entry of proliferating cells and regulate many aspects of mitosis necessary for successful cytokinesis, including centrosome duplication and maturation; DNA damage checkpoint activation; bipolar spindle formation; Golgi fragmentation and assembly; and chromosome segregation (Barr, F. A. et al., Nat. Rev. Mol. Cell Biol. 2004, 5, 429-441). PLKs are found in organisms as diverse as yeast and human and contain two conserved domains, the N-terminal catalytic kinase domain and a C-terminal region composed of the so-called polo-boxes. In yeasts a single PLK exist, whereas four distinct PLKs have been identified to date in mammals. Whereas PLK1, PLK2 and PLK3 are expressed in all tissues and structurally homologous in that they comprise the N-terminal catalytic kinase domain and two polo-boxes, PLK 4 differs not only in structure, compared to the other PLKs it has only one polo-box, but also in the distribution of PLK4 mRNA in adults that is restricted to certain tissues such as testes and thymus (Karn, T. et al., Oncol. Rep. 1997, 4, 505-510; Fode, C. et al., Proc. Natl. Acad. Sci. USA 1994, 91, 6388-6392). It is still under investigation whether these differences also result in a unique physiological role for PLK4.
Given the established role of PLKs as mitotic regulators, they have been regarded as validated mitotic cancer targets for a number of years. In addition, recent studies demonstrate that changes of intracellular levels of PLKs are involved in the control of cell growth. For example, PLK1 when fused to an antennapedia peptide and efficiently internalized into cells caused an inhibition of cancer cell proliferation (Yuan, J., et al., Cancer Res. 62, 2002, 4186-4190), whereas downregulation of PLK1 by antisense induced the growth inhibition of cancer cells (Spankuch-Schmitt, B., et al., Oncogene 21, 2002, 3162-3171). PLK2 was recently found to be a novel p53 target gene and RNAi silencing of PLK2 leads to mitotic catastrophe in taxol-exposed cells (Burns, T F., et al., Mol Cell Biol. 23, 2003, 5556-5571). For PLK3 it was found that it induces cell cycle arrest and apoptosis through perturbation of microtubule integrity (Wang, Q., et al., Mol Cell Biol. 22, 2002, 3450-3459) and PLK4 was shown to be transcriptionally repressed by p53 and induces apoptosis upon RNAi silencing (Li, J., et al., Neoplasia 7, 2005, 312-323). PLK4 was also found to be required for centriole duplication and flagella development. The absence of centrioles, and hence basal bodies, compromises the meiotic divisions and the formation of sperm axonemes (Bettencourt-Dias M., et al., Current Biology 15, 2005, 2199-2207).
All of this confirms that targeting PLKs with conventional small-molecule agents may be a valid and effective anticancer strategy with potential to synergize with established DNA-damage and antimitotic chemotherapies.
Relatively few reports of selective small-molecule PLK inhibitors have appeared to date.
Kyowa Hakko Kogyu has disclosed trisubstituted diamino-pyrimidine compounds as PLK1 inhibitors (PCT Int. Pat. Appl. Publ. WO 2004043936). Active compounds contain either a tetrazole group or a nitrile function at the pyrimidine C-5 position. A variety of (hetero)arylalkylamino groups are tolerated at C-2, whereas the C-4 substituent does not appear to be critical for PLK1 inhibitory activity.
A closely related pharmacophore, 5-nitro-N2,N4-diarylpyrimidine-2,4-diamines, was disclosed by GlaxoSmithKline (PCT Int. Pat. Appl. Publ. WO 2004074244). Again a variety of pyrimidine C-2 and C4 arylamine substituents were tolerated and the C-5 nitro group was a determinant of activity.
GlaxoSmithKline has also reported on a different pharmacophore, the 5-benzimidazol-1-yl-3-aryloxy-thiophene-2-carboxylic acid amides (PCT Int. Pat. Appl. Publ. WO 2004014899). Different substituents on the benzimidazole benzene ring were tolerated and a range of aryl ethers are present in the active compounds.
Onconova are developing amino-substituted 1(E)-2,6-dialkoxystyryl 4-substituted benzyl sulfones as cell-cycle agents with CDK inhibitory properties (PCT Int. Pat. Appl. Publ. WO 2003072062). Although reported as ATP-non-competitive kinase inhibitors, the relevant tumour cell target appears to be PLK1.
Certain macrocyclic compounds have been disclosed by the Applicants as inhibitors of tyrosine kinases (PCT Int. Pat. Appl. Publ. WO2004105765, WO2005058318, WO2005058913, WO2006061415 and WO2006061417) and as glycogen synthase kinase inhibitors (PCT Int. Pat. Appl. Publ. WO 2007003525).
It is accordingly an object of the present invention to provide selective small-molecule PLK inhibitors useful in the treatment of cell proliferative disorders.